The mechanism of carcinogenesis is studied with in vitro models to ascertain how mammalian cells respond to the various stimuli that are responsible for induction of transformation or that modify the frequency of transformation. The approach is to understand the nature of interrelationships between DNA metabolism, chromosome structure, biological reagents, and carcinogenesis. Human cells transformed by aflatoxin, UV, and by combination of X-ray and UV have been compared to non-treated cells and to human sarcoma specimens. The growth rate of all these cells was essentially the same; however, the chromosomal constitution varied from normal to minimal deviations to extensive chromosomal alteration for normal, experimentally transformed, and spontaneous sarcomas, respectively. Diploid fibroblasts from a patient with Bloom's syndrome were transformed by transfection with DNA from cells having single copy of the Harvey murine sarcoma virus (Ha-MuSV) per genome. Malignant characteristics of the cells and the identification by restriction enzyme analysis of the Ha-MuSV transforming sequence were evidence of the transformed phenotype. A comparison of effects of different methylating agents in hamster cells (HEC) indicates correlation of the transformation frequency with the level of 06- but not N7-methylguanine, determined by high pressure liquid chromatography. Sister chromatid exchange (SCE) frequency varies with growth conditions and/or class of carcinogen, indicating a relationship between SCE, unrepaired DNA damage, and transformation. The mechanism for SCE's is unknown, but protein synthesis appears critical and the level of spontaneous SCE is sensitive to changes in nucleotide pools. Lymphotoxin, an immunologic hormone, inhibits HEC transformations when added prior to or after carcinogen insult. The shorter the interval between lymphotoxin and carcinogen treatments, the greater the inhibition. Identification of DNA transforming sequences in guinea pig transformed cells is related to tumorigenicity. DNA transforming sequences have been identified in transformed HEC cell lines. Restriction enzyme analysis indicates several different sequences. One sequence, homologous to Kirsten ras, is being isolated.